[nova] Validation for requested host/node on server create

Matt Riedemann mriedemos at gmail.com
Thu May 23 22:02:15 UTC 2019

On 5/22/2019 5:13 PM, Matt Riedemann wrote:
> 3. Validate both the host and node in the API. This can be broken down:
> a) If only host is specified, do #2 above.
> b) If only node is specified, iterate the cells looking for the node (or 
> query a resource provider with that name in placement which would avoid 
> down cell issues)
> c) If both host and node is specified, get the HostMapping and from that 
> lookup the ComputeNode in the given cell (per the HostMapping)
> Pros: fail fast behavior in the API if either the host and/or node do 
> not exist
> Cons: performance hit in the API to validate the host/node and 
> redundancy with the scheduler to find the ComputeNode to get its uuid 
> for the in_tree filtering on GET /allocation_candidates.
> Note that if we do find the ComputeNode in the API, we could also 
> (later?) make a change to the Destination object to add a node_uuid 
> field so we can pass that through on the RequestSpec from 
> API->conductor->scheduler and that should remove the need for the 
> duplicate query in the scheduler code for the in_tree logic.
> I'm personally in favor of option 3 since we know that users hate 
> NoValidHost errors and we have ways to mitigate the performance overhead 
> of that validation.
> Note that this isn't necessarily something that has to happen in the 
> same change that introduces the host/hypervisor_hostname parameters to 
> the API. If we do the validation in the API I'd probably split the 
> validation logic into it's own patch to make it easier to test and 
> review on its own.
> [1] https://review.opendev.org/#/c/645520/
> [2] 
> https://github.com/openstack/nova/blob/2e85453879533af0b4d0e1178797d26f026a9423/nova/scheduler/utils.py#L528 
> [3] https://docs.openstack.org/nova/latest/admin/availability-zones.html

Per the nova meeting today [1] it sounds like we're going to go with 
option 3 and do the validation in the API - check hostmapping for the 
host, check placement for the node, we can optimize the redundant 
scheduler calculation for in_tree later. For review and test sanity I 
ask that the API validation code comes in a separate patch in the series.





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